Calculate Concentration Of Unknown Daily Reagent Biology Essay

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The purpose of this practical is to calculate the concentration of unknown daily reagent. Using the method of titration, vinegar and bleach will be examined the concentration.


In these two experiments, vinegar and bleach were used and the definitions of these two key chemicals are given. Vinegar, which results from the fermentation of ethanol; also, it is a widely used liquid (Conjecture Corporation, 2010). The main components are acetic acid (Conjecture Corporation, 2010). In addition, tartaric acid and citric acid may be comprised (Conjecture Corporation, 2010). Therefore, its pH measured as 2.4, which is strong acid (Lane, 2010). The main component of bleach is sodium hypochlorite (NaClO), and its common function is to sterilize.

The chemical reaction equation of the reaction of vinegar and sodium hydroxide can be given:


Through this acid-base reaction equation, it can be found out that when acid reacts to base, then salt and water will be produced. In addition, in this practical, the indicator (also called pH indicator), which is a substance will change its colour when the pH changes and it is widely used to show the completion of the acid-base reaction in titrations (Ch.ic, 2010). Indicators are usually weak acids or weak bases and they can dissolve in water and ions form (Ch.ic, 2010). Phenolphthalein indicator was used in this case, and it is a colourless liquid which is weak acid, and when it dissociates in water the pink anion will form (Ch.ic, 2010). When the phenolphthalein indicator meets acid solution, the colour will be colourless, and if it meets base acid, pink colour will be seen. The equation below shows the explanation about how the indicator works (Lister and Renshaw, 2000).

HIn(aq) H(aq) + In(aq)

Red Blue

If HIn and In are in different colour, one is red and the other one is blue, the HIn plays the role in an indicator (Lister and Renshaw, 2010 ). According to Lister and Renshaw (2000), as the prediction by Le Chatelier's principle, the equilibrium will be forced to the left when the concentration of H is high in a solution. The solution will be red as almost all the indicator exists as HIn. In a base solution where the concentration of OH is high, H ions will be moved away as HO (Lister and Renshaw, 2000).

HIn(aq) H(aq) + In(aq)

+ OH


As the prediction of Le Chatelier's principle, the equilibrium moves to the right side, and the majority of the indicator exists as In, the colour of the solution is blue (Lister and Renshaw, 2000).

Also, according to Lane (2010), the chemical equation of 'hypochlorite ion reacts with excess iodine ion in the presence of acid to generate an iodine solution can be given:

+1 -1 -1 0

ClO + 2I + 2H Cl + I + HO

In this equation, ClO is the oxygenant, and the I is the reductant.

In addition, the chemical equation of titration reaction can be given (Lane, 2010):

0 +2 -1

I + 2SO 2 I + SO

In this equation, I is the oxygenant, and the SOis the reductant.

In this experiment, the indicator is starch solution, which can alter the colour of the solution contains iodine ion. The colour is usually blue-black.


In this practical, vinegar, sodium hydroxide (NaOH) (concentration: 0.1 mol/dm), bleach, sodium thiosulphate solution (NaSO) (concentration: 0.1 mol/dm), potassium iodine (KI) (concentration: 1 mol/dm), sulphuric acid (diluted) (HSO), starch indicator and phenolphthalein indicator were used. Funnel, burette, test tubes, beakers, pipette, graduated cylinder and conical flask were used as the simple items during the experiment. Laboratory coats and safety glasses were used to ensure safety.

According to Lane (2010), these steps were done during the titration of vinegar experiment:

Firstly, a dropping pipette was used to drop a little NaOH and vinegar into two test tubes, and then 2 drop of phenolphthalein indicator solution were put into each test tube. The observation was written down. Secondly, the funnel was put on the top of the burette and the NaOH was poured into the funnel. Then the NaOH bottle was placed under the burette and some NaOH was poured back into the bottle until the level was read 0cm. The exact initial burette was read and written down. Thirdly, some vinegar was put into a conical flask. Some vinegar was poured into a burette and 2.5cm was obtained, and about 20cm water was poured into the flask. 4 drops of indicator solution was added into the flask. Fourthly, the flask was put under the burette, and the NaOH was started to add and the flask was shaken to mix the solution. This step was not stop until some colour change was noticed which disappeared as the flask was shaken. More NaOH was slowly added and the colour change lasted longer and longer. 1 drop of NaOH was dropped at a time when the end point was near. Finally, at the end point, 1 drop of NaOH would cause a colour change to last more than 20s, and the final burette reading was written down. After doing this experiment, the flask was washed out and the steps above were repeated.

According to Lane (2010), these steps were done during the titration of bleach experiment:

Firstly, a funnel was used to fill the burette with NaSO solution and it was adjusted to 0.1 in this case. The exact reading was written down. Secondly, 2cmof bleach was poured into a graduated cylinder, and 1cm was transferred to the conical flask. Thirdly, 10cmof the 1 mol/dm KI solution and 10 cmof diluted HSO solution were added to the conical flask, and the iodine was produced. Fourthly, the solution was added from the burette to the flask, and the flask was swirled continuously. Fifthly, a few drops of the starch indicator were added just before the end-point when the colour of the solution faded to pale yellow. The blue-black colour appeared and the thiosulfate solution should be added dropwise with thorough swirling. Finally, the end-point of the titration was detected when the blue-black colour changes to colourless. The burette reading was noted down. These steps were repeated for 3 times.


Table 1 shows the observations of the practical of the titration of vinegar.

Trial 1

Trial 2

Trial 3


Initial burette reading




When phenolphthalein indicator was added into the solution, there was no colour change.

When the indicator touched the surface of the solution, there was only a bit of amaranth occurred, after shaking, it disappeared.

When the end point reaches, the colour would not vanish when it was shaken.

Final burette reading




Volume of NaOH for neutralisation




Table 1

Table 2 shows the observations of the practical of the titration of bleach:

Trial 1

Trial 2

Trial 3


Initial burette reading




1. The original solution is dark brown.

2. When the NaSO solution was added, the solution turned to yale yellow.

3. When the starch solution was added, the blue-black colour can be seen.

Final burette reading




Volume of NaSO for neutralisation




Table 2


The following steps show the calculation to find the concentration of ethanoic acid:

The mole of NaOH = c (NaOH) * v (NaOH)

= 0.1 mol/dm * 26.5 * 10 dm

= 2.65 * 10 mol

∵ The mole of the H = the mole of OH

∴ The mole of ethanoic acid = the mole of NaOH = 2.65 * 10 mol

∴ The Concentration of = =

= 1.06 mol/dm

Therefore, this value of number is possible and expected.

In the titration of bleach experiment, the value which is used is the average of the three numbers.

∴ (4 + 4.1 +5) = 4.4 cm

The mole of SO = c (SO) * v (SO)

= 0.1 mol/dm * 4.4 * 10 dm

= 4.4 * 10 mol

According to the equations, the mole of the SO= the mole of 2 mole of the I = the mole of 2 ClO.

∴ The mole of ClO = * 4.4 * 10 = 2.2 * 10mol

∴ The Concentration of ClO = = = 0.22 mol/ dm

Therefore, this value of number is possible and expected.


Overall, the unknown valve of the concentration of a substance can be calculated when the other valve of the substance which reacts with it can be known. For instance, when the concentration and the volume of NaOH is known, the formula n=c*v can be used to calculate the mole of NaOH. Moreover, the mole of the acid equals the mole of the base. As a result, the unknown concentration of the ethanoic acid can be figured out.